The present invention relates to a method for stabilizing a fluorine-containing polymer. In particular, the present invention relates to a method for stabilizing a fluorine-containing polymer by treating a fluorine-containing polymer which has unstable chain ends and/or unstable bonds in the backbones under specific conditions.
In the case of, for example, emulsion copolymers of tetrafluoroethylene and hexafluoropropylene, bubbles or voids may be formed from volatile materials in products produced by melt processing. The volatile materials are generated from the unstable chain ends and unstable backbones of the polymers, when heat or shear force is applied to the polymers.
The kinds of unstable chain end groups vary with polymerization methods, and the kinds of polymerization initiators and chain transfer agents. For example, carboxylic acid terminal groups are formed, when a conventional persulfate salt (e.g. ammonium persulfate, potassium persulfate, etc.) is used as a polymerization initiator in emulsion polymerization. It is known that such carboxylic acid terminal groups are the sources for volatile materials in the melt processing. Depending on the conditions in the melt processing, groups such as olefinic groups (xe2x80x94CFxe2x95x90CF2), acid fluoride groups (xe2x80x94COF) and the like are formed at the chain ends. These end groups may cause bubbles or voids in the final products of the polymers.
Backbones which may generate volatile materials may be bonds between comonomers other than tetrafluoroethylene (TFE), as U.S. Pat. No. 4,626,587 describes. In the case of tetrafluoroethylene-hexafluoropropylene copolymers (FEP), the unstable bonds in the backbones are bonds between hexafluoropropylene monomers (HFP). This is confirmed form the fact that, when a gas generated by heating and melting FEP around 400xc2x0 C. is analyzed, a molar ratio of HFP to TFE in the generated gas is about two times larger than that in the polymers.
U.S. Pat. No. 4,626,587 proposes the removal of unstable chain end groups and unstable bonds in the backbones, which may be the cause of bubbles or voids found in the final products of fluorine-containing polymers, by the application of a shear force with a twin-screw extruder. However, the use of a twin-screw extruder can remove the unstable bonds in the backbones because of the large shear force of the extruder, but hardly stabilizes the unstable end groups because of the too short residence time. In addition, it is very difficult to remove coloring which appears because of the severe melting conditions, and the residues of polymerization initiators or contamination. Thus, additional stabilization treatment such as fluorination with other equipment is necessary after the treatment with the twin-screw extruder. Furthermore, molded articles should be treated at a temperature lower than the melting point of the polymer, when the unstable end groups are stabilized after melt molding, since the shapes of the molded articles should be maintained.
An object of the present invention is to provide a method for effectively removing unstable end groups and unstable bonds in backbones from fluorine-containing polymers and also coloring, in the melt kneading step.
The above object can be achieved by a method for improving the thermal stability of a fluorine-containing polymer comprising melt kneading a melt-processable fluorine-containing polymer with a kneader which has a residence time of at least 10 minutes, a usable volume ratio (usable space in a container/space in a container) of larger than 0.3, and a power factor K of less than 8000, the power factor K being represented by the formula:
K=Pv/xcexc/n2
wherein Pv is a power requirement per unit volume (W/m3), xcexc is a melt viscosity (Pa.s), and n is a rotation speed (rps).